In recent years, coping with environmental problems, such as global warming, and resource conservation have become imminent issue in human society. Therefore, continuous research and development efforts have been made to save fuel and energy in automobiles, machines, apparatus, including industrial machines, etc. The role of lube oil employed in such machines and apparatus is basically to attain stable operation of the machines and apparatus, but demand has arisen to reduce wear and friction to thereby enhance a fuel saving effect.
One known effective means for saving fuel cost is reducing viscosity of lube oil. For example, when the viscosity of lube oil employed in an automatic transmission (AT) having a torque converter, a gear bearing mechanism, a hydraulic mechanism, a wet clutch, etc. is reduced, fluid resistance (stirring resistance) of the members is reduced, conceivably lowering fuel cost.
However, when the viscosity of lube oil is lowered, the lube oil is prone to vaporize, and as a result, evaporation loss increases. This also causes an increase in viscosity of the lube oil during operation.
In addition, reduction of the viscosity of lube oil decreases the fatigue life of such machines. Specifically, metal fatigue such as scoring or spalling occurs at a gear bearing mechanism and other friction parts, and lubrication characteristics such as extreme pressure characteristics are impaired. Particularly, since the sizes and weights of ATs have decreased and torque capacity has increased in recent years, gear bearings receive an increased load. Also, since automobiles of an AT of larger number of gear positions such as a 6-speed AT have increased, a gear (planetary pinions) is operated under high-speed rotation, which causes high-speed friction against a bearing. Thus, metal fatigue and lubrication characteristics have become severe problems.
Furthermore, a transmission fluid is required to have good oxidation stability.
One example of such a conventional transmission fluid whose viscosity is reduced so as to save fuel cost is a transmission fluid produced through blending a base oil having a naphthene content and an aromatic content controlled to fall within specific levels with a specific extreme-pressure agent (see, for example, Patent Document 1). However, such a lube oil exhibits a large evaporation loss and has other problems. Thus, such a lube oil is required to be further improved.    Patent Document 1: Japanese Patent Application laid-Open (kokai) No. 2004-262979